The present invention relates to a wide symmetrical lens and, more particularly, to a symmetrical, ten element lens particularly suited for use in a photocopier optical system with document semi-field viewing angles greater than 40.degree..
In electrophotographic copiers, the areas of a charged photoconductive surface, which are irradiated by a light image, are discharged, the degree of discharge dependent upon the intensity of the impinging light rays. It is therefore desirable that the light intensity vary only due to the reflectance characteristics of the original document being copied rather than due to changes introduced by the imaging components. Stated in another manner, the optimum system would be one that provides uniform photoconductor irradiance given a uniformly reflecting document.
Of the factors affecting relative illumination at an imaging plane, the most significant is the cosine (cos) variation wherein the irradiance at an image plane is approximately proportional to the cos.sup.4 of the field angle. Thus, even if an object plane is uniformly illuminated, photoconductor irradiance decreases as radial distance from the system optical axis increases. Various approaches have been devised to compensate for this effect. Typically, in systems where a document is incrementally illuminated and scanned by a lamp/mirror assembly, a sheet of opaque material having a butterfly-shaped slit formed therein is used to increase the amount of illumination at the edges of the image.
In full frame imaging systems wherein an entire document is illuminated by flash illumination, efforts to compensate for cos.sup.4 light falloff have emphasized location the flash lamps in such a way that the document edges are illuminated to a greater degree than the central areas. Such a system is disclosed in U.S. Pat. No. 3,777,135. If the full frame flash system provides a magnification (generally reduction) option, a variation of the field angles introduces further cos.sup.4 variations with magnification changes and it has proved difficult to maintain the desired image plane uniformity through all the different magnification positions with this lamp arrangement compensation method.
Another method of cos.sup.4 compensation is to smooth the exposure by reducing center illumination by means of a relative illumination filter. Such a filter design for a full frame flash copier is disclosed in U.S. Pat. No. 4,298,274. This solution has the disadvantages of requiring additional components (the filter) in the imaging system and also of requiring additional illumination power to compensate for the light transmission loss through the denser areas of the filter.
The cos.sup.4 illumination falloff is an approximation which neglects the effects of pupil aberration. Coma of the pupil which is introduced by the optics between the object and aperture stop, can cause the pupil to increase or decrease in size for off-axis object points. A decrease in pupil size means that the aperture stop itself vignettes the off-axis light bundle and reduces the irradiance more than the cos.sup.4 falloff. An increase in pupil size will allow more light to pass through the lens off-axis if the outer elements are large enough to pass the increased bundle.
A lens such as the Topogon form disclosed in U.S. Pat. No. 2,031,792 has pupil coma which reduces the off-axis pupil size in such a way as to cause approximate cos.sup.5 fall off. This effect is shown graphically in the Schneider publication "Technical Information G-Claron WA."
It would be preferable to compensate, in a wide angle system, for the cos.sup.4 effects without the use of additional components such as additional flash lamps or relative illumination filters and without requiring increases in system illumination or, conversely, with no decrease in illumination efficiency. Such compensation has been suggested by various lens designs used for the purpose of aerial photography, as disclosed in U.S. Pat. Nos. 2,516,724; 2,730,016; 3,154,628; 2,821,113; and 4,331,391. The latter two patents disclose non-symmetrical designs requiring aspheric surfaces; characteristics which are not suitable to flat document copying requirements. Further, none of the references discloses a wide angle lens which provides compensation through a magnification range.
The present invention is directed to a novel wide angle lens whose design partially compensates for cos.sup.4 illumination light fall off, thus requiring less electrical energy consumption. More specifically, the invention relates to a wide angle photocopy lens system having a semi-field angle greater than 38.degree., a relative aperture of F/10, said lens system being partially compensated for cos.sup.4 relative illumination fall off through a magnification range of 0.64 to 1.55.